Giardia lamblia is the most common protozoan cause of diarrhea in the world. It infects ~500 million people worldwide, often resulting in nutrient malabsorption which can lead to physical and cognitive developmental defects in children. Recent data suggest that the parasite is able to glycosylate secreted and surface proteins with N-acetylglucosamine and that recognition of this sugar by the mannose binding lectin (MBL) can activate the complement cascade. Our hypothesis is that recognition of parasite glycoproteins by host lectins plays a significant role in activating both innate and adaptive immune responses. We will address this specifically by examining infections in mice lacking host lectins and examining the impact on the course of infection and the innate and adaptive immune responses.